This invention relates to an improved process for the production of linear alpha-olefins by oligomerization of ethylene. More particularly, this invention is directed to an improvement in the recovery of diol reaction solvent in a process wherein ethylene is oligomerized by contact with a catalytic nickel complex dissolved in an aliphatic diol reaction solvent.
Linear monoolefins are compounds of established utility in a variety of applications. Terminal linear monoolefins, particularly those having 12 to 20 carbon atoms per molecule, are known to be useful as intermediates in the production of various types of detergents, e.g., alcohols, ethoxylates, etc.
Several synthetic techniques have been developed for the preparation of terminal linear monoolefins in the detergent range. One very attractive synthetic method from the standpoint of raw material availability and cost involves oligomerization of ethylene to higher molecular weight linear monoolefins (even numbered alpha-monoolefins) by contact with a catalytically active nickel complex dissolved in certain polar solvents. One class of suitable nickel complex catalysts for ethylene oligomerization is prepared as the reaction product of an olefinic nickel compound, including zero-valent nickel compounds such as bis(cyclooctadiene) nickel (O) or .pi.-allyl nickel compounds, and a suitable bidentate ligand as described in U.S. Pat. No. 3,644,564 to Van Zwet et al, U.S. Pat. No. 3,647,914 to Glockner et al and U.S. Pat. No. 3,647,915 to Bauer et al. A different and preferred class of nickel complex catalysts can be prepared by contacting in certain polar organic solvents in the presence of ethylene (1) a simple divalent nickel salt which is at least somewhat soluble in the solvent, (2) a boron hydride reducing agent and (3) a suitable bidentate ligand. The preparation of catalysts in this preferred class and their use in ethylene oligomerization are described in U.S. Pat. Nos. 3,676,523, 3,686,351 and 3,737,475 to R. F. Mason and U.S. Pat. No. 3,825,615 to Lutz.
In cases where the oligomerization is carried out using the preferred nickel complex catalysts in a polar organic solvent, preferably an aliphatic diol, the reaction product typically consists of three phases: (1) a liquid solvent phase in which catalysts are dissolved; (2) a liquid hydrocarbon phase which consists of the total oligomer and includes dissolved ethylene, solvent and nickel complex catalyst and (3) gaseous ethylene. In early attempts to recover the oligomer product from this three-phase reaction product by a series of phase separations and flashing or distillation steps, it was discovered that the small amounts of residual catalyst present in the liquid hydrocarbon phase promoted the formation of objectionable, polymeric polyethylene when catalyst, solvent and ethylene are present in the hydrocarbon product phase at conditions under which part of the hydrocarbon phase is removed by flashing or distillation. As one means of preventing the formation of polyethylene, U.S. Pat. No. 4,020,121 to Kister and Lutz discloses a stepwise process for recovery of active catalyst, polar reaction solvent, gaseous ethylene and ethylene oligomers from the oligomerization reaction product in which the liquid hydrocarbon product phase is subject to a scrubbing step using additional polar organic reaction solvent prior to the time that the catalyst-contaminated hydrocarbon phase is subjected to depressurization for removal of ethylene. In general terms, the overall recovery process described in the aforementioned U.S. Pat. No. 4,020,121 includes an initial degassing step wherein entrained ethylene gas is separated from the two liquid components of the oligomerization reaction mixture for direct recycle to the oligomerization zone followed by phase separation of at least part of the solvent phase from the degassed liquid to afford a liquid hydrocarbon phase substantially free of solvent. According to the patent teaching, the separated liquid hydrocarbon product phase is subsequently passed to a product scrubber where it is contacted with a stream of pure oligomerization reaction solvent under sufficient pressure to avoid flashing of dissolved ethylene, said solvent serving to remove residual active catalyst from the hydrocarbon phase. After removal of the residual active catalyst, the separated hydrocarbon product is passed to a deethenizer for removal of dissolved ethylene and the deethenized product is water-scrubbed to remove residual, dissolved or entrained solvent thereby affording an oligomer product essentially free of solvent, catalyst and ethylene. In the process scheme described in this reference, the bulk of the polar reaction solvent phase containing active catalyst from the liquid-liquid phase separation is suitably recycled to the oligomerization zone with the remainder of the separated solvent being passed to a solvent recovery zone in which purified solvent is produced. This solvent recovery zone is suitably comprised of a fractionation column in which light end impurities and spent catalyst are removed thereby affording a purified reaction solvent which is advantageously employed to scrub catalyst residue from the hydrocarbon phase in the product scrubber (see above) or as a solvent source in the preparation of additional catalyst.
While the processing scheme described in the aforementioned U.S. Pat. No. 4,020,121 provides an attractive means of recovering ethylene oligomers from oligomerization reactions employing nickel complex catalysts in polar organic solvents, it is not completely free of problems. One area of difficulty involves the solvent recovery zone wherein reaction solvent is separated from light end impurities and spent catalyst. In particular, it has been found that when aliphatic diols are employed as the source of polar organic solvent in the oligomerization reaction, the conditions required to separate solvent from the spent catalyst in the solvent recovery zone also promote conversion of the diol solvent into a series of oxygenated degradation products. These oxygenated contaminants which are typically oxidized and/or condensed derivatives of the diol solvent (carbonyl compounds, acetals and hemiacetals) have boiling points and solubilities sufficiently similar to the produced oligomers that they are very difficult to remove from the oligomer product if the recovered solvent is recycled to the oligomerization process. For example, when a preferred oligomerization solvent such as 1,4-butanediol is employed, a series of tetrahydrofuran-type impurities are formed in the solvent recovery zone which have solubilities and boiling points quite similar to the oligomer product. Thus, unless these oxygenated impurities are somehow removed or the recovered solvent is not revised in the process, the impurities will appear as contaminants in the final oligomer product in cases where the oligomers are recovered directly or, they may act as catalyst poisons in cases where the oligomer product, or a portion thereof, is subject to further processing such as sequential isomerization and disproportionation described in U.S. Pat. No. 3,766,939 to Berger.
From the foregoing, it is apparent that an advantage could be obtained if the oligomerization solvent recovery could be somehow modified to substantially eliminate the diol solvent degradation products as a source of oligomer product contamination. Further, it would be particularly desirable if the formation of diol solvent degradation products in the solvent recovery zone could be avoided or minimized with minimal process expense and equipment modification.